The present invention relates to a piston ring, and more particularly, to a piston ring formed of a base material having an outer layer that is first selectively subjected to localized and superficial heating such that the outer layer is transformed to an austenitic form, and second to a rapid cooling such that said austenitic form is converted to a martensitic form.
Piston rings are well known. They are usually received within an annular groove disposed about an outer periphery of a piston. In turn, the piston is reciprocated within a cylinder of an internal combustion engine. Typically, the piston ring is discontinuous, having two end portions. The end portions are separated from one another to expand the piston ring for insertion into a corresponding groove of the piston. The piston ring is then compressed, bringing the end portions closer together, to install the piston within the cylinder.
A piston compresses fluids such as gases within the cylinder. In an internal combustion engine, these fluids are ignited and expand, forcing the piston away from the point of ignition. The outer surface or bearing face of a piston ring in an internal combustion engine is subjected to high temperatures, corrosion, and frictional interaction with the walls of the cylinder.
To improve durability, wear and scuff resistance, it is known to use a variety of wear surface materials bonded to an underlying substrate forming the core of the piston ring. Examples include wear surface materials comprising ceramics, nickel-boron face coatings and various nitride coatings such as chromium nitride coating. These wear materials have a number of limitations. First, they are extremely brittle and subject to cracking or other unacceptable failure modes. Second, there is often a problem with achieving a sufficient coating thickness such that the coating does not prematurely wear away and expose the base material, thereby possibly resulting in piston ring failure. Third, it is extremely difficult to machine a piston ring having a known wear material already bonded to the base material. Fourth, piston rings with known wear materials are often heavier, increasing inertial force and friction within the cylinder. Finally, known piston rings with wear materials bonded to a base material are typically fairly expensive to manufacture.
Another way to improve durability, wear and scuff resistance of piston rings is to form the piston ring from a base material having a first hardness, and then subject the base material to a heat treatment, thereby hardening the base material. Typically, piston rings of this type are formed from powdered, austenitic iron, and are then subjected to a form of heat treatment to form martensitic iron in at least a portion of the piston ring. However, austenitic iron is difficult to work with, and forming piston rings from powdered austenitic iron may create undesirable manufacturing defects within the piston ring. Moreover, the entire piston ring is subjected to a heat treatment, requiring sufficient time and heat treatment to affect substantially all of the piston ring.
The present invention is directed to a piston ring for use in a piston of an internal combustion engine. The ring includes a base material of a first hardness formed into a ring, and a secondary material of a second hardness formed by selective and superficial heat treatment of an outer portion of said base material. The heat treatment may be selectively and superficially limited to only the face and contact areas of the piston ring, or may involve the entire surface of the piston ring.
In a first embodiment, the ring is formed from cast iron, iron-aluminum alloys or steel alloys. At least a portion of the outer surface of the ring is selectively and superficially heat treated in one of a variety of ways to form an austenitic metal layer about 25-200 xcexcm, and most preferably 100 xcexcm thick on a surface of the ring. The ring is then rapidly cooled in an appropriate environment, resulting in the transformation of the austenitic compound into a fine martensitic compound adjacent the base material at the point of heat treatment. Preferably, the martensitic metal has a hardness range of 800 to 1000 HV (Vickers Hardness scale) measured at a load of 100 grams. In a second embodiment, the base material is a quenched and tempered alloy that includes coarse martensite. The heating of a portion of the surface of the base material causes a transition to austenite, and the subsequent rapid quenching transforms the austenite into a fine martensite that is harder than the original base material.
Several methods may be used to heat the surface of the piston ring. In one method, electrons emitted by a cathode are accelerated and concentrated onto an outer surface of the piston ring while it is in a high vacuum environment. As the electrons interact with the surface of the ring, energy is transferred to the surface of the ring, thereby heating the surface of the ring. In this way, the electron beam heats the surface of the ring beyond the temperature of austenite formation.
In another method, the surface of the ring is heated by focusing a laser upon the surface of the ring such that the temperature of the surface of the ring exceeds the temperature of austenitic iron formation. In other methods, the surface of the ring is heated by an induction furnace or by exposure to a plasma torch.
In all cases, the base material comprising the piston ring is relatively ductile and workable, selected from the group comprising cast iron, aluminum-iron alloys or steel alloys. The base material must be able to transition to both an austenitic form and a martensitic form having the desired end characteristics described below. In a first embodiment, the base material has not been hardened or pre-treated in any way, and need not include any austenite. In a further embodiment, the base material has been quenched and tempered so that it includes some martensite. In both embodiments, the base material is relatively easier to initially form and shape. The heat treatment according to the present invention occurs only superficially on the piston ring, so the entire ring is not subjected to heat treatment. In fact, because all of the methods described for heating the surface may be selectively aimed, portions of the surface of the piston ring may be selectively hardened. Thus, the heat treatment may be limited only to the face or contact areas of the piston ring. As a result, the original ductility of the base material is sufficiently maintained to allow conformability of the piston ring with the groove of a piston.
The surface heat treatment also occurs over a relatively short period of time, compared to the time required to heat treat an entire piston ring. By limiting the scope of the heat treatment, the time required to heat treat the piston ring is reduced, resulting in manufacturing efficiencies. Manufacture time is further reduced without adversely affecting the piston ring quality. Finally, because the heat treatment occurs only to a limited depth on the piston ring, only those faces which require hardening need be subjected to the treatment, thereby maintaining the ductility and resiliency of the base material.